This invention relates to a loudspeaker system effecting sound directivity, in which a plurality of speakers are disposed in a suitably spaced relation for causing a strong sound pressure in a specific direction by mutual interference of acoustic waves.
Conventionally, a loudspeaker system of a Tonsaule type in which a plurality of speakers, e.g., four speakers, are arranged in a straight line as shown in FIG. 6, have been well known as a speaker system for effecting sound directivity.
With the Tonsaule type speaker system as shown in FIG. 6, the speakers SP are aligned in a line with a predetermined distance d between each and other speakers. Thus, as shown in FIG. 3, the sound pressures from two speakers SP cancel each other out to produce a zero pressure at a point P.sub.90 in a direction of 90.degree. relative to a point P.sub.0 on the center axis, such cancellation being due to the fact that the phase difference between the two sound pressures is 180.degree. out of phase at the point P.sub.90 at a frequency f.sub.0 and having wavelength of which is given by d=.lambda./2.
At the point P.sub.0, the sound pressures strengthen each other to cause a peak value of the sound pressure in the P.sub.0 direction. At a point P.sub..theta. between the point P.sub.0 and the point P.sub.90 the sound pressure is of a value between the peak value and the zero pressure, with the sound pressure decreasing gradually toward zero pressure with increasing angles of direction toward P.sub.90, thus resulting in a directive pattern indicated by a solid line curve as shown in FIG. 4.
However, with sound at a frequency having a wavelength d=.lambda., the sound pressures strengthen each other at the point P.sub.90, causing a directive pattern as shown by a dotted line in FIG. 4.
For the reasons described above, the Tonsaule type speaker system exhibits, as shown in FIG. 7, a directivity pattern depicted by a solid line A at the mid frequency range and by a dotted line B at lower frequency range, thus causing or effecting an inadequate sound directivity.
Further, the Tonsaule arrangement is further disadvantaged in that it is required that the speakers be disposed or distanced in a spaced relation of d=.lambda./2, which leads to a large overall size of the system.
In addition to the Tonsaule type speaker, a parametric speaker has also been in practical use but it has not been widely used because it suffers from problems in that an ultrasonic modulation device is required, and reproduction of the sound at the lower frequency range is inherently difficult.